Database

db={"pipe":[{_id:1,deal_id:25698},{_id:2,deal_id:45879},{_id:3,deal_id:54142}],"leads":[{_id:1,name:"Teste A",deals_id:[25698,45879]},{_id:2,name:"Teste B",deals_id:[54142]}]}

Query

db.pipe.aggregate([{$lookup:{from:"leads",localField:"deal_id",foreignField:"deals_id",as:"results"}},{$unwind:"$results"},{$project:{_id:1,deal_id:1,name:"$results.name"}}])

Result